1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and more particularly, to a developing device used with an electrophotographic image forming apparatus, which supplies a developing agent to a photosensitive body and develops the photosensitive body, and a developing method thereof.
2. Description of the Related Art
In general, electrophotographic image forming apparatuses form an electrostatic latent image which corresponds to a desired image by radiating light on a photosensitive body, form a toner image by supplying toner to the electrostatic latent image, transfer the toner image onto a recording medium, and fuse the toner image on the recording medium, thereby printing an image.
FIG. 1 is a cross-sectional view schematically illustrating a conventional electrophotographic image forming apparatus. Referring to FIG. 1, the conventional electrophotographic image forming apparatus includes a photosensitive body 1, a charging roller 2, an exposure unit 3, a developing unit 4, a transfer roller 9, and a cleaning unit 11.
The charging roller 2 supplies a charge to a surface of the photosensitive body 1 and charges the surface of the photosensitive body 1 at a predetermined electric potential. The exposure unit 3 radiates light corresponding to image information on the photosensitive body 1. Then, an electrostatic latent image is formed by a difference in electric potential between a first portion on which the light is irradiated and a second portion on which the light is not irradiated. The developing unit 4 forms a toner image by supplying toner to the electrostatic latent image.
The developing unit 4 includes a developing roller 5 which is spaced—apart from the photosensitive body 1 by a predetermined gap, a supplying roller 6 which supplies the toner to the developing roller 5, and a layer regulation member 7 which regulates a thickness of the toner attached to a surface of the developing roller 5.
A developing electric field Vd is applied to the developing roller 5 so as to supply the toner to the electrostatic latent image formed on the photosensitive body 1. The developing roller 5 is rotated in a direction opposite to the photosensitive body 1. That is, outer circumferences of the photosensitive body 1 and the developing roller 5 progress in the same direction at a developing nip where the photosensitive body 1 and the developing roller 5 face each other.
The supplying roller 6 is rotated while contacting the developing roller 5. The supplying roller 6 is rotated in the same direction as the developing roller 5. That is, outer circumferences of the developing roller 5 and the supplying roller 6 progress in different directions at a contact portion between the supplying roller 6 and the developing roller 5. A supply electric field Vs is applied to the supplying roller 6 so as to attach the toner to the developing roller 5.
The layer regulation member 7 contacts the outer circumference of the developing roller 5 while applying a predetermined pressure thereto. The layer regulation member 7 regulates the toner attached to the outer circumference of the developing roller 5 so as to have a predetermined thickness, and simultaneously rubs against the toner, thereby charging the toner.
The toner image is transferred onto a sheet of paper passing through a space where the transfer roller 9 and the photosensitive body 1 are located. Reference numeral 12 denotes a fusing unit which fuses the toner image onto the paper by applying heat and pressure to the paper passing along a path 13. The cleaning unit 11 removes the toner remaining on the surface of the photosensitive body 1 after a transfer operation.
According to the above structure, the developing electric field Vd and the supply electric field Vs are applied to the developing roller 5 and the supplying roller 6, respectively, during a printing operation so that the toner can be attached to the developing roller 5.
The toner remaining on the surface of the developing roller 5 after the developing roller 5 has developed the electrostatic latent image on the photosensitive drum 1, is partially detached from the surface of the developing roller 5 by friction with the supplying roller 6. And, the toner remaining on the surface of the developing roller 5 may be stuck onto the surface of the developing roller 5 while passing through a space between the developing roller 5 and the supplying roller 6 repeatedly. In particular, when the remaining toner passes through a space between the developing roller 5 and the layer regulation member 7 repeatedly, a frictional heat accumulates to increase a temperature thereof so that the toner may be fused onto the layer regulation member 7. In order to obtain a uniform image concentration, a thin and uniform toner layer should be formed on the outer circumference of the developing roller 5. However, if the toner is stuck onto the surface of the developing roller 5 and/or the layer regulation member 7 as described above, the uniform toner layer cannot be formed. In addition, if the remaining toner passes through the space between the developing roller 5 and the layer regulation member 7 repeatedly and rubs against the layer regulation member 7 to increase a charging amount, new toner is prevented from being supplied to the developing roller 5. Thus, the new toner is not sufficiently supplied to a partial region of an image, and thus, spots or stripes occur on a printed image, and an image quality is lowered.